1. Field of the Invention
The present invention relates to the use of coherent energy processes for high powered pulse lasers, in the shock processing of materials, and more particularly, to methods and apparatuses for determining if sufficient energy has been applied to a workpiece to work the part.
2. Description of the Related Art
Known methods of shock processing solid material, in particular laser shock processing solid materials using coherent energy, as from a laser, orient the laser beam normal, i.e., perpendicular to the workpiece.
Laser shock processing techniques and equipment can be found in U.S. Pat. No. 5,131,957 to Epstein.
Problems arise during production, in particular there is difficulty in ascertaining whether the laser peening process has applied sufficient irradiance to correctly work the part. Particular questions to be answered for part quality are that of the amplitude and duration of the pressure pulse applied to the workpiece.
It is difficult to test the processed workpieces, to determine if sufficient pressure has been applied, without destruction of the workpiece.
Previous process quality issues in similar process metal shot peening, have been determined with the use of Almen strips or test coupons. These types of metallic test coupons are in the form of strips formed with a known composition and structure. Such test coupons are standardized in composition, hardness, and thickness. The test strip is placed against a steel block and the entire exposed surface is processed with the appropriate shot peening intensity to be tested. The residual stress introduced by shot peening causes the coupon to arch, and such arch is calculated to be related to the intensity of the peening.
Such test strips or coupons have proven to be unsatisfactory for laser shock peening in that the strips have a large surface area. If the entire surface is laser peened, the test is time consuming and costly. If only a part of the surface of these strips is processed, the sensitivity of the arching to different laser peening intensities is low and not reproducible.
What is needed in the art, is an apparatus or method for directly measuring the pressure pulse generated by the laser beam or a material response relating to laser peening intensity, that can be utilized for each shot or at intervals during laser shock processing. These methods or apparatus should be inexpensive and provide rapid measurement having acceptable accuracy.
The present apparatus and method is that of a quality control device whereby, periodically during production processing utilizing laser shock peening, the device is inserted into the beam or beams or alternatively monitors the effects produced by each laser shot. The laser is shot at the inventional device instead of the workpiece, to obtain a readout of whether or not the laser peening system is operating within the correct processing range. The system measures the characteristics of the pressure pulse created by the laser beam, not the laser beam itself.
The present invention includes the opportunity to directly measure the pressure or impulse created by the laser peening system by a plurality of methods. One system utilizes a material sensor utilizing the piezoelectric effect. In this case, the pressure pulse passing through the material creates a particular electric response which can be measured and correlated to the applied pressure pulse. Examples of these materials are quartz, lithium niobate and some polymers such as polyvinylidene fluoride (PVDF).
Another possible means of directly measuring the pressure pulse is to utilize materials displaying piezoresistance effects. Examples of these types of materials are manganese, carbon and ytterbium.
Still another method is to utilize fiber optic materials which show a change in refractive index with pressure.
Another type of direct measurement of the pressure pulse may be some type of pressure sensor that may be able to withstand the applied pressure of the laser peening system. Additionally, such measurement systems may include attenuating material that enables reuse of the sensor and/or may be connected to other sensing devices by fluid, such as air, liquid, or solid connections.
Another feature of the invention is that it has the ability of sensing vibrations and elastic waves created by the pressure pulse by using an electronic strip, such as an acoustic sensor or microphone directly attached to the workpiece, to measure the acoustic waves created by the shockwave of the pressure pulse.
For measurement of acoustic signals or responses, the voltage generated by the passage of the pressure pulse through the sensing device may be traced upon an oscilloscope, and such data may then be digitized and saved to create an effective acoustic signature of what would believed to have been a sufficient pressure pulse.
In the above cases, the pressure pulse measured would have to pass through the current operating method of the laser peening system, particularly that being of the appropriate transparent overlay and opaque overlay such that the pressure response, at the measuring device, may be correlated directly to the pressure pulse expected at and to the workpiece.
Another method of measuring the pressure pulse would be to use an indirect measurement, such as a microphone connected to the workpiece, or located in the laser shocking area, or a microphone attached to the workpiece holding tool. Such non-contact sensors may include the use of microphones, a laser acoustic measurement device measuring surface shock waves on the workpiece, or alternatively, bouncing a separate laser beam (at a different wavelength than that of the laser peening system) off the piece and measuring movement of the workpiece surface or vibration caused by the reflected waves. It may be necessary in some types of applications to create a curved adaptor to fit between the microphone and the workpiece, such as a flexible bellows, or some other type of conforming accommodating apparatus to maximize the contact area. Such contact connection between the measuring device and the workpiece may be made by a clamp, adhesive, or other contact device.
The invention, in one form thereof, is a method of testing the operation of a laser peening system, comprising providing a sensor in a possible laser beam path, applying a transparent overlay material to the sensor; and directing a pulse of coherent energy to the sensor through the transparent overlay material to create a shock wave. The system then determines a characteristic of the created shock wave with the sensor, or measures the affects of the shock wave on the workpiece.
An advantage of the present invention is that it would be reusable between particular laser shock peening parts to ensure that the pressure pulses applied to subsequent workpieces are substantially the same.
Another advantage of the present invention, is that by the use of pressure sensitive devices that may be calibratable, a more accurate reading of the pressure pulses created by the laser peening system is possible. In addition, since the devices may be used more than once, they can be recalibrated after a certain amount of use to further increase their precision.
Another advantage of the present invention, is that the system may have direct or indirect contact with the workpiece.
Yet another advantage of the present invention is the use of nondestructive evaluation techniques in combination and controlling the laser peening system.